Method of strengthening glass by ion exchange and article made therefrom



United States Patent O Y 3,287,201 METHOD F STRENGTHENING GLASS BY ION EXCHANGE AND ARTICLE MADE THEREFROM Raymond S. Chisholm, Pittsburgh, George E. Sleighter, Natrona Heights, and Fred M. Ernsberger, Pittsburgh, Pa., assignors to Pittsburgh Plate Glass Company, Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 7, 1963, Ser. No. 249,790 8 Claims. (Cl. 161-1) This invention is directed to improved glass articles and methods of preparing such articles. More specifically the present invention is directed to improving the strength of glass, particularly lime-soda glass such as is used in the production of window or plate glass.

According to the present invention, increases in strength are secured in glass articles by first contacting glass with an inorganic compound of an electropositive element such as lithium or hydrogen of smaller atomic diameter than the predominant alkali metal in said glass at a temperature high enough to introduce this element into the glass without developing cracks or weakness defects in the glass surface, generally at a temperature above the strain point of said glass, and thereafter contacting said glass with a salt of an alkalitmetal of larger atomic diameter than said electropositive element at a temperature below the strain point of said glass. This process is especially useful because it may be applied to the low cost commercially available soda-lime glasses such as plate and window glass. Recourse to glasses of special composition containing high concentrations of lithium or like relatively expensive batch materials is not a requisite of this invention.

The invention will be further understood with reference to the accompanying drawing in which:

The ligure of the drawing is a graphical representation showing a typical stress profile of glass treated in accordance with Example I of this invention.

As shown in the drawing, many of the glass articles of the present invention. are characterized by a surface compressive stress zone, a central interior tensile stress zone and an additional tensile stress zone underlying said surface compressive stress zone yet exterior to said central interior tensile stress zone. This stress profile is present on both major surfaces of the glass articles of this invention, as can be seen from the ligure of the drawing. The maximum tensile stress in said underlying or intermediate tensile stress zone is greater than the maximum tensile stress in said central interior tensile stress zone, and the ratio of the maximum compressive stress in said surface compressive stress zone to the maximum tensile stress in the central interior tensile stress zone is always atleast 10 to l, usually 1000 to 1 and greater, and sometimes greater than 10,000 to 1. The ratio of the maximum tensile stress in said underlying tensile stress zone to the maximum tensile stress in said central interior tensile stress zone is always at least to l, generally at least 100 to 1, and sometimes as high as 500 to l and even higher.

The central interior tensile stress zone contains an alkali metal, usually sodium or potassium, substantially in the concentration characteristic of the given glass composition subjected to treatment. In the case of soda-lime glass having the normal composition of plate or window glass, tests have shown that the underlying tensile stress zone contains a concentration of the electropositive element of smaller atomic diameter, e.g., lithium, which concentration is greater than that of the central interior tensile stress zone and may be preponderant to the concentration of the preponderant alkali metal of the central zone. The surface compressive stress zone contains a molar preponderance of an alkali metal, e.g., sodium and/ ice or potassium, of larger atomic diameter than said lithium or like smaller electropositive element of said underlying tensile stress zone.

The preponderant alkali metal in the surface compressive stress zone can be of the same atomic diameter as the preponderant alkali metal in the central interior tensile stress zone, or the preponderant alkali metal in the surface compressive stress zone can be of larger atomic diameter than the preponderant alkali metal in the central interior tensile stress zone. Thus, the alkali metal in the surface compression zone and the base or central zone may be sodium.

Glass articles of this invention exhibit unique optical stress profiles in that a compression layer exists at the surface (surface compressive stress zone), Whereas the zone underlying the surface compressive stress zone is in tension (underlying tensile stress zone) which tension decreases rather sharply in magnitude as the distance from the outer portion of the underlying tensile stress zone increases so as to approach a nominal tension towards the mid-plane of the glass article (central interior tensile stress zone). The central interior tensile stress Zone has a nominal tension, viz., a tensile stress which is approximately the same as or even slightly below that of conventional annealed glass, viz., 20 to 150 pounds per square inch. Of course, the corresponding stress profile exists from the center of the glass sheet toV the opposite surface, namely, the stress profile goes from the nominal tension of the central interior zone existing through the center to increased tension at the opposite underlying tensile stress zone and then sharply into high compression at the surface compressive stress zone which exists at the opposite surface thereof.

Glass of the type produced according to this invention is stronger and is less subject to failure under impact loads than untreated glass.

The thickness of the surface compressive stress zone for a given surface generally ranges from about 10 to about 200 microns and even thicker, usually being at least 40 microns in thickness. The underlying tensile stress zone is generally thinner than and seldom exceeds the thickness of the compressive stress zone. Usually the underlying tensile stress zone ranges from about l0 percent to percent of the thickness of the surface compressive stressV zone. The central interior tensile stress zone occupies the main central portion of the glass article as measured from the mid-plane of the glass article moving outwardly to both exterior surfaces.

One of the significant beneficial characteristics of the high strength glass articles provided in accordance with this invention is their ability to be cut readily Without shattering. In this regard they differ markedly from ordinary thermally tempered glass.

Moreover, thin glass articles treated in accordance with the present invention not only possess a high surface strength but also are surprisingly flexible. For example, lime-soda-silica glass articles 0.090 inch thick when treated in accordance with the procedure of Example I (lithium treatment followed by sodium treatment) are suiliciently flexible to be bent around a circle having a 30 inch radius without fracture of the glass.

To some extent at least the strengthening of glass in accordance with this invention involves a selective series of metal atom exchange procedures. As a consequence 0f the first exchange treatment, which involves the exchange of the electropositive element of smaller atomic diameterfor the larger preponderant alkali metal present in the base glass, there may be imposed a tension stress for a depth penetration measured by the depth of exchange of the smaller diameter electropositive element for the predominant alkali metal(s) present in the glass composition. However, in many cases the magnitude of the stress is small when the glass is cold and as disclosed below the surface may even be under compression under these circumstances.

VThe later atom exchange treatment or treatments are conducted by exposing the treated glass to an exchange ofV an alkali metal of larger atomic diameter than the Ysaid smaller electropositive element of the rst treatment. This Ylater exchange treatment or treatments imposes a compressive stress on the outermost surface of the glass Y article fork a depth measured by the depth for which the larger atomic diameter alkali metalis exchanged for the smaller atomic diameter electropositive element.

The first exchange procedure is conducted at elevated temperatures to insure adequate penetration of the smaller diameter electropositive element in a reasonably rapid i period of time andavoid breaking orffracturing thersurface of the glass. The useV of elevated temperatures allows the stresses imparted to the glass by the rst exchange procedure to Vbe at least partially thermally relaxed especially at the outer surface, thus minimizing the likelihood of cracking. To accomplish thesefobjectives generally the rst exchange procedure is conducted at temperatures above the strain point of the chosen glass composition. However, Vit should 'be realized that it is'within the purview of this invention to conduct they Y irst exchange procedure at lower temperaturesnover longer periods while avoiding or` at least effectively minimizing surface stress cracking.

The later ion exchange treatment or'treatments are conducted at a temperature below the strain point ofthe glass so as to avoid thermal relaxation of the compressive stresses at the time they are being imposed upon the glass by the exchange of the larger-atomic diameter alkali metal for the smaller atomic diameter electro-` positive element.

This invention is applicable to 'a wide range of glasses containing in excess of 40 percent'by Weight of SiO2 and if desired such other glass formersyas boron and aluminum inthe oxide form as well as variousalkali metal and alkaline earth metal oxides.

Lime-soda glasses of theY type contemplated normally contain at least about l0 percent sodium, determined as NazO, and at least about 5 percent calcium, determined as CaO, as well as 65 to 75 percent SiO2. A representative range of composition in which lthe respective element content is determined as their respective oxides is as follows:

Percent by weight NagO l0 to 20 KZO 0 to 5 CaO 3 to 15 SiO2 65 to 75 MgO Oto 10 B203 0 to 5 A1203 5 Some of the Nag() in the above table can be replaced by KZO. l l

A typical soda-lime glass suitable for use in accordance with this invention has the following composition:

' Percent by Vweight SiO 71.38 (usual variation While the present invention will be illustrated hereinafterby discussion primarily relating to monolithic glass Y plates and sheets, it should be realized that the basic effectl of the practice of the present invention is to increase su-bstantially the lscproe of utility of glass to include its useY` where high-strength properties and surface compression properties are' advantageous ina myriad of fabricated 'Y Hence, the value of the present articles of commerce. invention "extends not onlyto such articles as yviewingV closures, windshields, backlites and sidelites, but also to other glass articles such as those used in thecontruction and building fields and all of the fields where materials are required to possess high strength proporties, eg., architecturalY spandrels; windows; doors; bottles; glass tablewaree.g., cups, saucers, plates; skylights; etc.

-This invention lis especially advantageous .because it,

is capable of strengthening ordinary soda-lime glass, e.g.,

window, plate or bottle glass, by providing it with a surface compression layer of unusual depth.

Treatment o f such glass, for example, by immersion` of the glass in molten potassium nitrate to exchange aV larger atom, e.g., potassium from the treating salt for the sodium in the glass can be performed in order to improve the strength ofthe glass and impart a surface compression thereto. However, the depth of the glass under compression normally is quite hollow, rarely exceeding 10 to 20 microns even after very long immersions. having such shallow surface portions under compression are subject to abrasions such asgscratches which, potentially, can be deep enough to penetrate substantially., such surface portion glass..

1n contrast, by practicing the present invention surface compression zones which exceed 10 to 20 microns and -range from 30 to 200 microns can bek obtained readily and without diculty. Thus, ordinary glass of low cost can be substantially strengthened and provided with `a durable surface portion which is under compression to such a depth as to decrease Vthe likelihood of loss of glass strength due to scratches and other abrasions occasioned by ordinary wearY and tear. Y

A further advantage is that the `glass thus obtained has durability not substantially diiferent from thatfofr ordinary sodalime glass. This is particularly true when sodium isexchan-gedfor the smaller atom in the ksecond treatment since in this case the gross chemical composi tion (and perhapsteven atomic orientation) of the treated glass does not differ substantially from that of the original untreated glass.

Prior to the rst exchange treatment, viz.,YV treatment L with asa-1t of an electropositive element Ahavin-g a smaller .atomic diameter than ,the predominant alkali metal in the .chosen glass composition, it is found preferable to Iheat the glass article to be treated to a temperature within a range of 50 F. above or 'below the temperature at which the firstV exchange treatment is to be'conducted,

viz., the temperature at which the yexchange treating bath is maintained during treatment. Preferably the Iglass f article i-s preheated to a temperature fairly closely ap.-` proxirnating that at which the -iirst exchange treatment is conducted. By this means surface cracking of the glass and undue chilling of the first treating bath areV avoided.

In a typical method of performing this invention the preheated glass, for example, window or plate -glassl in sheet form, is dipped in a :molten bath of a salt of lithium,

such as lithium sulfate. The temperature of `the bath should be high enough to avoid or minimize formation of cracks or defects as a consequence o'f the treatment. Thus, the treatment tends to cause introduction of lithium into the glass, e.g., by a replacement of lithium for the sodium of the glass and thereby to develop a surface tension in thek glass which can produce cracks.

ture ystresses -cerated bythe'introduction of lithium are relaxed enough to prevent cracking.

The exact temperature of the dipdepends uponthe properties of the base glass. -Fo'r lime-soda glass, temperatures a'bove 800 normally are required, a suite Glass thus reducing the strength of the.`

By con-` ducting the lithiumktreatment at a lhigh enough tempera-` `able temperature being 1000 to 1l00 15. or other temperatures above the strain point o'f the glass. The temperature rarely is above that at which the tglass is molten.

A convenient .period of contact between the lithium salt and the glass is about 5 to 20 minutes, although longer times of immersion, for example, several hours, can be employed. The result is to produce a glass in which the lithium content of the surface has been enhanced, i.e., the lithium concentration of the surface portion is substantially above (more than twice) that of the interior. The depth of thie enhancement of lithium concentration is above l to 20 microns. Where the temperature of treatment is not high, this surface may be under a small tensile stress. When the glass is cooled to room temperature, it may be essentially unstressed. I'f the treatment is conducted ata 'high temperature so that alll tensile stress produced by the lithium is relaxe-d, the glass when cooled may have its external surface under compression.

The glass havin-g this enhanced lithium content on the surface is then immersed in molten sodium nitrate or like molten sodium salt at a temperature below the strain point or at least below about 1000 F. for a period of time ranging from minutes u-p to several hours or even longer with a period of 5 to 30 minutes usually being sufcient. Longer periods of immersion are -not objectionablle so long as the surface compressive stress induced 'by the treatment is retained. The result is to deplete the lithium -on the surface and thereby to regenerate a surface which, While it -is under compression, does not differ materially in chemical composition from that of the base 'glass since exchange of the lithium initially introduced for sodium takes place to a depth of 20 to 200 microns or even deeper depending upon the depth of the initial lithium penetration.

In lieu of a sodium'salt, a molten potassium salt can be used to replace lithium lwith potassium thereby producing glass of even greater strength. Also molten mixtures of sodium and potassium salts may be used.

The-salts used should be relatively stable at the temperatures of operation. Typically satisfactory salts are the sulfates, nitrates, chlorides, uorides, and .phosphates of the above metals, lithium, sodium, potassium and the like.

Another advantageous embodiment of this invention involves the strengthening of lime-soda-silica glass by a three-step series of exchange treatments. The rst exchange treatment 4is conducted lby immersion of a glass article into a bath of molten lithium salt at a temperature above the strain point of the glass. The second exchange treatment is conducted by immersing the thus treated glass article into a lbath of molten sodium salt at a temperature below the strain point of the lglass, eg., temperatures ranging from about 750 to 850 F. whereby the sodium concentration on the surface is increased, and the lithium concentration yis decreased. The third exchange procedure is conducted by immersing the thus treated glass into a bath -of molten potassium salt maintained at a temperature below the strain point of the glass, e.g., temperatures ranging from Iabout 750 to 850 F. Inthis step potassium replaces at least a portion of the sodium in the outer surface portions of the glass.

Such successive treatments, all of which yare preferably conducted at temperatures below the strain point of the glass, can serve to increase the thickness and compressive strength of the surface compressive stress zone.

Usually the Iglass is cooled between each exchange treatment to a temperature roughly approximating room temperature, viz., a temperature ranging from 200 F. down to and even below room temperature. Following cooling the glass is usually subjected to aqueous rinsing prior to subsequent exchange treatments.

In place of a first lithium exchange treatment, limesoda-silica glass can be first treated with a hydrogen salt to substitute hydrogen for sodium, for example, by subjecting the glass to a molten acidsalt, such as sodium acid sulfate or the like. Hydrogen treatment can also be effected by treating the glass with an acid, e.g., sulfuric acid. Generally the acid will be volatilized at treatment temperatures used in the first exchange. Or instead of either lithium or hydrogen, other salts of an electropositive element(s) having an atomic diameter smaller than sodium (the predominant alkali metal in lime-soda-silica glass) can be used as long as said salts effect an exchange of their electropositive element(s) at a temperature above the strain point of the particular lime-soda-silica glass composition chosen for treatment.

The term atomic diameter as used herein means the crystal angstroms atomic diameter as expressed in kilo- X, viz., kX units. These kX units are smaller than absolute angstroms, and kX=A/ 1.0020. The term atomic diameter as used herein denotes the kX as reported on pages 20-23 (column `4) of the booklet Key to the Welch Periodic Chart of the Atoms (1959) by William F. Meggers. The atomic diameter values in said booklet were mainly taken from The Structure of Metals and Alloys by William Hume-Rothery, The Institute of Metals, London, 1945.

The following examples serve to illustrate the invention in greater detail. However, it should be understood that the invention in its broadest aspects is not necessarily limited to the particular materials, temperatures, treatment times and other conditions set forth below in the examples.

Example l The glass subjected to treatment is conventional plate glass which has the following composition:

Polished flat glass plates 4 inches by 4 inches by 0.125 inch of this glass are preheated to a temperature of 1050 F. plus or minus 20 F. in an insulated oven over a period of approximately 15 minutes.

Then the preheated plates are directly immersed into a molten lithium salt bath which is at a temperature of 1050 F. and contains 80 mole percent lithium sulfate and 20 mole percent potassium sulfate for a period of 10 minutes to effect exchange of lithium for sodium. In this immersion treatment, the molten lithium salt bath is contained in a fused lined 2000 milliliter cylindrical stainless steel beaker of 8 inches height, approximately 1/32 inch wall thickness and having an inner diameter of approximately 5% inches. The fused liner is a preformed cylinder sold under the designation Vitreosil by the Thermal American Fused Quartz Company, and is 8 inches in height and has an inner diameter of approximately 5 inches and a wall thickness of approximately 0.5 inch. The fused silica liner contains 99.9 to 99.93 percent by weight silica, SiO2, with alumina, A1203, representing two thirds of the total impurities.

The fused silica liner is tted into the stainless steel beaker. Then the lithium salt is placed into the fused silica liner and heated to the treatment temperature (1050n F.). The sample glass plates are mounted vertically on a stainless steel rack which carries live sample plates each spaced approximately 0.250 inch on each major surface from an adjacent plate. The rack containing ve sample plates is then placed into the molten lithium salt bath to completely immerse all plates therein. The molten lithium salt bath is maintained at a sufficient depth, approximately six inches, to allow complete immersion of each sample.

VAt the end ofthe ten minuterimmersion, the rack and lithium treatedplates are removed from the moltenY lithium 4bath and gradually cooled to room temperature. Then the lithium treated plates are rinsed with water to remove excess lithium salt'and dried. y

These lithium treated plates, which have their surfaces under compression, and whose surfaces contain lithium introduced by exchange to a depth of approximately 100 microns, are preheated to a temperature of approximately 800 F. over a ten minute period, and then totallyimsurface compressive stress zone, the underlying tensile Y ystress zone and the central interior tensile stress zone.

The samples are also tested for load breaking strength.

The optical stress measurements, both tensile stress and compressive stress, are measured by birefringence using a graduated quartz wedge (prism) looking through 0.020 inch sections of the 0.125 inch thick treatedglass plates. UsingY a diamond cutting wheel, a 0.020 inch section is cut with the nearest cut being no less than 0.5 inch from the sample edge. The 0.020 inch section is then mounted on a microscope side so that the 0.020 inch dimension is normal to the surface of the microscopeslide. Then index matching fluid, viz., oil having thesame index of refraction as the glass section, is placed over the glass surface. The section, thus mounted on the microscope slide, is then placed on the stage of a petrographic microscopel (one containing the polaroids built into the optical system below the stage surface) equipped with a quartz` wedge. The quartz wedge is calibrated in millimicrons. The sample to be measured is viewed by looking at the 0.125 inch surface through the 0.020 inch dimension,kviz.,

the polarized light passes through the 0.020 inch dimen-V sion. The zone of the sample which is to be measured is selected and the stress of that zone is measured by look- Y ing through that zone. The compressive stress at the surface (compressive stress zone) is measured by looking through the 0.020 inch surface at either edge of 0.125 inch dimension. TheY tensile stress of the central interior tensile stress zone is measured by looking through the 0.020 inch surface at the center of the 0.125 inch dimension. The

tensile stress Vof the additional tensile stress zone (intermediate or underlying tensile stress) is measured by looking through the 0.020 inch surface at a location `on the 0.125 inchdrimension which is between the center and either edge but closer to` either edge, e.g., approximately 50 to 150 microns from either edge of the 0.125 inch surface. Since the sample section is 0.020 inch thick, the measured values are multiplied by 50 Yto express the stress in terms of millimicrons per inch. Then the optical rating of tensile stress in millimicrons per inch isKconverted to mechanical pounds per square inch stress units by multiplying by the stress optical coeflicient of 2.13, which applies not only for the glass composition listed above in this example but also for most conventional plate and sheet glass. In Table I below the optically determined stress valuesare set forth in pounds per square inch.

The load strength tests are conducted using concentric ring loading on the four-inch square 0.125 inch thick i test plates. The larger circular ring has an internal diameter of 3 inches whereas the smaller circular ring has an internal diameter of 1.5 inches. Both concentric rings have knife edges-which contact the glass surfaces ina direction normal to the 4 square inch area thereof. The load speed is 0.02 inch per minute, and the reported load strength is the average load strength (pounds per square inch) at which failure (glass breakage) occurs for the tested samples, all of which areidentically treated for strengthening. This load strength in pounds per square inch Vis arrived at by multiplying the actual load lstrength (pounds) by the stress conversion factor, which is 34.88, which applies to 0.125 inch thick plate glass Within the realm of experimental error.

The load strength and optical stress data are summarized in Table I below in comparison with corresponding values of 4 inch by 4 inch by 0.125.inch control samples of polished iiat Vglass plates of identical glass composition but having no strengthening treatment.

Example Il In this test the glass used is sheet glass and has the':

following composition: Y

Composition A Component: (percent by weight) SiOg 71.35 NazO 13.24 KZO 0.03 CaO `11.76` MgO 2.41 A1203 0.12 Na2'SO4 0.53` Fe203 0.53

Polished glass sheets 4 inches hy4 inches by 0.125 inch of the composition listed above are provided. V'Ihe sheets are preferentially exchange treated first with lithium, Vthen with sodium as in Example I to improve the strength thereof. The average optical stressesand load strength of` these sample sheets are essentially the same as the corresponding values of the strengthened pla-tes of Example I.

Example III Polished lime-soda-silica at glass plates 4 inches by 4 inches by 0.125 inch of the composition listed above in Example I are provided. These plates are preferentially exchange treated first with lithium, then with sodium zas in Example I. Then these plates are preheated to 800 F. and subjected to a further compressive stress treatment by immersion in a molten potassium nitrate salt bath at 800 F. for immersion periods ranging from l5 minutes to 8 hours followed by removal, gradual cooling to room temperature, aqueous rinsing and drying.

As a result of this potassium treatment both the average optical compressive stress and average load strength are improved over that attained using 'the two step lithiumsodium exchange treatment of Example I.

The total thickness of the surface` compressive stress zone is essentially unchanged from Vthat recorded in the sample plates of Example I, but the outermost portion (3 to 30 microns) of the surface compressive stress zone, which contains a potassium content which yis above that of internal portions of the glass primarily due to potassium exchange for sodium, possesses a higher compressive stress than the remaining intermediate 20 to 47 microns of the surface compressive stress zone inY which the alkali metal is predominantly sodium primarily due to sodium` exchange for lithium.

The average thickness and tensile stress magnitude of the underlying tensile stress zone is essentially unchanged from that recorded in conjunction with the sample plates of Example I. The same is true of the central interior tensile stress zone.

The difference between the maximum and minimum compressive stress per unit thickness in the potassium induced upper portion (3 to 30 microns) of the surface compressive stress zone (measured from the bottom thereof to the outer surface) is greater than that in the sodium induced lower portion of the compressive stress zone (measured from the interior to exterior thereof). That is to say the slope of the compressive stress curve (plotting magnitude of compressive stress yagainst depth of the zone) in the potassium induced (upper) portion of the surface compressive stress zone is steeper than that in the sodium induced (lower) portion of the surface compressive stress zone.

Depth of the potassium exchanged portion of the surface compressive stress zone is essentially directly proportional to the length of the immersion period during which the plates are treated with molten potassium nitrate, viz., the longer the potassium treatment, the deeper the potassium exchange for sodium at a given potassium treating temperature.

Exam-ple IV Polished at glass plates 4 inches by 4 inches by 0.125 inch of the composition listed in Example I are preheated to a temperature of 1050 F. and directly immersed in a molten (1050 F.) lithium salt bath for a period of 10 minutes to etect 'exchange of lithium for sodium following the same treating procedure indicated in Example I. After lithium treatment these plates are removed from the lithium bath and gradually cooled to room temperature. These plates are then washed with water and dried.

Then the lithium treated plates are heated to a temperature of approximately 800 F. over a period of 15 minutes after which the heated plates are directly immersed into a molten salt bath of potassium nitrate at 800 F. for 4 hours, After removal from the molten potassium bath, these plates are allowed to cool gradually to room temperature, and are subsequently rinsed with water and dried.

-Optical stress and load strength determinations reveal substantial increases in optical compressive stress and load strength compared to untreated plates of identical glass composition and size.

Example V Polished at glass plates 4 inches by 4 inches by 0.125 inch of the composition listed above in Example I are preheated to a temperature of 1050 F. plus or minus 20 F. in an insulated oven over a period of approximately 15 minutes.

Then the preheated plates are directly immersed into a molten sodium bisulfate bath at a bath temperature of 1050 F. for a period of 10 minutes to etect exchange `of hydrogen from the sodium bisulfate salt for sodium present in the glass. At the duration of the minute immersion period, the sample plates are removed from the molten sodium bisulfate bath, and gradually cooled to room temperature. The hydrogen treated plates are then Washed in water and dried.

These plates are then heated to a temperature of approximately 800 F. over a period of 10 minutes and then totally immersed -in a hath of sodium nitrate at a temperature of 800 F. for l5 minutes. After removal of the sodium treated plates .from the molten sodium nitrate salt bath, the plates `are cooled gradually to room temperature, Washed with Water to remove excess sodium salt, and dried. After drying, the plates :are subjected to optical stress measurement and load strength testing as in Example I. Optical stress measurements reveal the presence of a surface compressive stress zone averaging 50 microns in depth and of a magnitude approximating that secured in Example I above. Beneath the surface compressive stress zone there yis an underlying tensile stress zone averaging approximately 40 microns in thickness and having a maximum tensile stress of the same order as that obtained in Example I. The average central interior tensile stress of the hydrogen-sodium strengthened plates is slightly below that of conventionally annealed plate glass.

Example VI over a period of 15 minutes followed by direct immersionA into a molten potassium nitrate salt bath at 750 F. for 8 hours. After removal of the hydrogen-sodium-potassium treated plates from the potassium nitrate bath, they are allowed to cool gradually to room temperature. The plates are then Washed with -water and dried. Optical stress measurements and load strength testsare then performed on these plates. From these measurements it is apparent that both the optical surface compressive stress and load strength of the hydrogen-sodium-potassium treated plates are greater than the corresponding values obtained for the hydrogen-sodium strength treated plates of Example V. Both the thickness and magnitude of the underlying tensile stress zone of these plates are essentially the same as recorded above for the plates of Example V. The same its true of the central interior tensile stress of these plates as compared with those of Example V.

The benefits and advantages attendant to the method and articles of the present invention are Igenerally applicable to glass articles regardless of their thickness. It is especially advantageous when applied to thin -glass which is diiicult to thermally temper, such as glass ranging from approximately 0.060 inch and below. However, thicker glass articles, viz., articles having thicknesses of 1A inch all the way up to extremely thick -glass articles such as structural glass articles, e.g., Iglass doors and the like, may lbe subjected to treatment as contemplated herein,

In the production of the articles of this invention over extended periods of time, when a plurality of glass articles are successively immersed in the various molten alkali metal treating baths, the composition of the baths must be `controlled to prevent alkali attack upon the glass surfaces being treated. For example, in the rst stage of the exchange treating process, wherein the glass plates are treated -with an electropositive element having a smaller atomic diameter than the predominant alkal-i metal present in the glass, it was noted that unless the build-up of acidity and/ or alkalinity in the lithium salt treating bath was neutralized, either acid or alkali attack on the glass resulted. For this purpose the molten lithium salt bath is neutralized with either potassium Ibisulfate or SO2 gas to combat a build-up of alkali in the lithium treating bath. Of course an acid build-up is neutralized by use of any suitable alkaline material and in simil-ar fashion. For this reason, when the first ion exchange treatment is conducted using a molten lithium salt hath, the composition of the molten bath is controlled so that the mole percent of Iavailable hydroxyl groups (alkali) is :below -about 0.005 percent. In similar fashion the mole percent of hydrogen (acidity) of the molten lithium salt -bath is maintained below about 0.0005 mole percent. The alkaline vor acid accumulated build-up in the molten lithium salt bath is determined periodically by removing a sample from the molten lithium salt lbath, cooling the sample, dissolving it in water and titrating using a 0.1 N acid or l 1 tbase titrating solution containing appropriate indiactor. The sample is'then titrated until appearance of the neutral color is achieved by titration.

The subsequently employed molten alkali metal sal n baths (using alkali metals having larger atomic diameters than the electropositive element employed during the first exchange treatment) present similar problems as far ask acid and alkaline contamination as well as build-up yof exchanged smaller atomic diameter alkali metals in these Itreating baths.V These baths' are mainained using similar precautions indicated above in conjunction with the molten lithium salt bath. However, the later exchange treatments using the larger atomic diameter alkali metal salt baths do not require as close control in maintenance of limitations on -basicity and acidity as the molten lithium salt bath employed for the first exchange treatment;

For example, when lime-soda-silica glass is first ex- Y change treated with lithium -followed by later exchange treatments yby Aimmersion in molten sodium nitrate and then immersionin molten potassium nitrate, the sodium content of the molten potassium nitrate bath increases. In" general for uniform treating results in the potassium treatment stage of this process, the sodium content of the i bath isheld 'below 10.0 percent by weight and preferably below 5.0 percent by weight based upon the combined weight of sodium and potassium in the bath. Most preferably the sodium content is maintained at a level ranging :enough temperature, yglass which is under surface compression when cooled is obtained. Also any surface de- Y fects which may have`r developed in the course` of the treatment can be healed by the heating. Y

Following this the second exchange treatment can be I.

conducted as hereinabove disclosed. v

'While the various exchange `treatments can be conducted effectively by immersion of the glass in a molten bath, other methodsof treatment can be used. Thus,A the glass can be sprayed or otherwise coated ywith a coat-V ing of the lithium salt and the glass heated to a tempera-` ture at which the lithium saltis molten, cfg., to 1050 F.v This step may be performed as an incident to bending' downwardly from 1.0 percentV by weight to a value approaching and even reaching 0 percent lby weight. In such a treatment procedure the sodium content of the molten potassium treatin-g bath should not be permitted to vary more than 5, and preferably less than 2 percentV by weight (based upon the total combined weight Vof sodium and potassium in the molten potassium bath) from the early`` (low sodium content) stage of immersion to Vthe latter (higher sodium content) stage thereof, even though pluralities of glass articles are dipped over a period of 1 to 20 weeks. In order to counteract the build- The present invention can ybe employed to produce glass articles which are also subjected to conventional processing techniques, such as thermal tempering (preferably performed subsequent to the first exchange treatment and prior to the later exchange tre'atment(s); cutting operations; laminating operations; etc., to produce glass articles having enhanced surface-strength, impact resistance and penetration resistance.

As will be noted from the foregoing, the present invention is by no means limited to glass sheets but can be employed to produce windshields, building materials,V

architectural glass, bottles, drinking glasses, tableware, viewing closures such as window panes, safety -glass and other laminated structures, glassy insulation structures wherein a plurality of glass sheets are arranged in spaced fashion with a layer of air serving as the insulation medium, television safety glass implosion and/or explosion shields, glass roofs or .transparent domes in vehicles and buildings, experimntal devices such as glass engine parts which must withstand a highcompression, ceramic and siliceou-s articles used in the-dental art such as dentures and crown caps, ceramic mulers, for automobiles, airplanes and other vehicles, etc.

According to a further embodiment, the lithium or like treatment can be conducted below the strain point of the glass. Thereafter the glass can be heated above its strain point to relax tensile strains therein, for example, at a temperature of 1000 F. to 1300 F. If this heat treatment is conducted for a long enough time and high ment of surface cracks.

the glass. Glass thus coated can be placed on bending irons andpassed through the usual bendingcycle, care being taken to hold the glass at a high temperature (for example,` above the strain point) forY a long enough period to ensure penetration ofthe lithium. The glass is then bent and exchange treated in the same operation.

This bent glass is cooled and dipped in the moltenV sodium or potassium salt as described above.

Furthermore, thesodium or potassium treatment can be effected by coating .the glass with` the salt and heating to a suitable temperature, for example, 700 to 900 F.

As shown in Example I, a lithium composition comprising mole percent lithium sulfate and 20 mole percent potassium sulfate is used in the first treatment. This',

is done to enable use ofra lithium salt bath which isf molten at temperatures below the thermal deformation temperature of soda-lime glass and to minimize develop- Byuse of a salt composition in which the lithium content is below that of the pureV neutral salt, probability of such cracks being produced is` reduced. `Heat stable salts which do not etch theV glass` other than sulfates, including the chlorides, uorides, phosphates, nitrates and the like can be used in lieu of.'` sulfates in the above process so long as the temperatures are held low'enough to avoid an undesirable degree of` decomposition of the salt and/or attack and impairment of the surface of the glass.

prior to the subsequent sodium and/or potassium treatment.

While the above examples illustrate the use of sodium and/ or potassium alkali metal salts for the later exchange treatment of lithium `or hydrogen exchange treated-sodalime glass, it should be realized thatsuch later exchange,

treatment can be yconducted using inorganic salts of other alkali metals above the'electropositive element used for the first exchange treatment in the atomic series viz., an

Even if some such surface: attack takes places in the deposit of the lithium or like atom, it may be healed to a degree by heat treatment` inorganic salt of an alkali metal having an atomic num- Thus, e.g., lithium, copper, cesium,

the later exchange treatment of soda-lime glass `which has been first exchange treated withhydrogem Correspondingly, lithium treated glass can be subjected to a later exchange treatment using,'e.g., inorganic saltsV of copper, Vcesium, rubidium or silver.

Although the present invention has been described with respect to specific details of certain embodiments thereof, it is not intended that such details serve as'limitations ber larger than the electropositive element of the `first Yexchange treatment.

rubidium and silver inorganic salts can be employed forl 1 upon the scope and spirit of the present invention.V The present invention in its broadest aspects is not necessarily above the strain point of the glass, reducing the temperature of the glass ybelow the strain point after said replacement, and thereafter replacing said smaller electropositive metal ions with larger electropositive metal ions selected from the group consisting of the ions of alkali metals, copper and silver by contacting the glass with v a source of said larger electropositive metal ions while retaining the temperature of the glass below the strain point of the glass until the surface is placed in compression by said latter replacement.

2. The method of claim 1 wherein the selected larger and smaller electropositive metal ions are alkali metal ions.

3. The method of claim 1 wherein the selected smaller electropositive metal ions are hydrogen ions.

4. The method of claim 3 wherein the replacement of alkali metal ions in the glass by hydrogen ions is accomplished by contacting the glass with a molten acid sulfate at a temperature above the strain point of the glass.

5. The method of claim 1 wherein the glass is a limesoda-silica glass and the smaller electropositive metal ions are lithium ions.

6. The method of `claim 1 wherein the strengthening steps are followed by `cutting the strengthened article into smaller strengthened portions.

7. A strengthened glass article produced according to the process set forth in claim 1.

8. A strengthened glass article having a stress pattern substantially as shown in the single ligure in the drawing.

References Cited by the Examiner UNITED STATES PATENTS 2,154,490 4/ 1939 Burch 65-31 2,779,136 1/ 1957 Hood et al. 65-30 3,107,196 10/ 1963 Acloque 65-115 3,218,220 11/1965 Weber 65-111 OTHER REFERENCES Kistler, Stresses in Glass Produced by Nonuniform Exchange of Monovalent Ions, J. of Amer. Cer. Soc., vol. 45, No. 2, pp. 59-68, February 1962.

DONALL H. SYLVESTER, Primary Examiner.

S. LEON BASHORE, Examiner.

G. R. MYERS, Assistant Examiner.

Notice of Adverse Decision in Interference In Interference No. 95,963 involving Patent No. 3,287,201, R. S. Chisholm, G. E. Sleghter and F. M. Ernsberger, METHOD OF STRENGTHEN IN G GLASS BY ION EXCHANGE AND ARTICLE MADE THEREFROM, final judgment adverse to the patentees was rendered May 26, 1969, as t0 claims 1 2 5 and 7.

, [Oficial Gazette November 25, 1969.] 

1. A METHOD OF STRENGTHENING AN ALKALI METAL CONTAINING GLASS WHICH COMPRISES REPLACING THE ALKALI METAL IONS IN A SURFACE OF THE GLASS BY SMALLER ELECTROPOSITIVE METAL IONS SELECTED FROM THE GROUP CONSISTING OF THE IONS OF ALKALI METALS, COOPER, SILVER AND HYDROGEN BY CONTACTING THE GLASS WITH A SOURCE OF SAID SMALLER ELECTROPOSITIVE METAL IONS WHILE RETAINING THE GLASS AT A TEMPERATURE ABOVE THE STRAIN POINT OF THE GLASS, REDUCING THE TEMPERATURE OF THE GLASS BELOW THE STRAIN POOINT AFTER SAID REPLACEMEN, AND THEREAFTER REPLACING SAID SMALLER ELECTROPOSITIVE METAL IONS WITH LARGER ELECTROPOSITIVE METAL IONS SELECTED FROM THE GROUP CONSISTING OF THE IONS OF ALKALI METALS, COPPER AND SILVER BY CONTACTING THE GLASS WITH A SORUCE OF SAID LARGER ELECTROPOSITIVE METAL IONS WHILE RETAINING THE TEMPERATURE OF THE GLASS BELOW THE STRAIN POINT OF THE GLASS UNTIL THE SURFACE IS PLACED IN COMPRESSION BY SAID LATTER REPLACEMENT.
 8. A STRENGTHENED GLASS ARTICLE HAVING A STRESS PATTERN SUBSTANTIALLY AS SHOWN IN THE SINGLE FIGURE IN THE DRAWING. 